1. Field of the Invention
The present invention relates to an optical pickup and optical read/write drive that writes data on an optical storage medium and that reads data that is stored on an optical storage medium. More particularly, the present invention relates to an apparatus that carries out verification on data being written on an optical storage medium while performing the write operation. Examples of optical storage media that can be used in the present invention include optical tapes, optical discs and optical cards.
2. Description of the Related Art
Recently, the size of digital data that can be stored on a storage medium has been rising steeply year by year as the resolutions of video data and still picture data have been tremendously increased and as increasing numbers of paper media have been converted into electronic ones. Meanwhile, so-called “crowd computing” technologies that allow people to use various kinds of applications and services via servers and storage systems on some network have become more and more popular nowadays. According to such crowd computing technologies, as a huge number of users save various kinds of data on that storage system on the network, the amount of data accumulated should keep on skyrocketing from now on.
In the meantime, as regulations have been established one after another with regard to the duty of preserving such a huge amount of data saved, it should also be increasingly important to devise a method for saving that enormous amount of data as securely and as reliably as possible.
An apparatus that writes data of such a huge size on an optical storage medium must perform the operation of seeing if the data has been written just as intended on the optical storage medium in order to increase the reliability of writing. Such an operation will be referred to herein as a “verify operation”. In this description, an “optical storage medium” will refer to a medium on which a mark can be recorded optically when irradiated with a light beam. And the light beam is radiated from an “optical pickup” that includes a light source and a lens that focuses the light beam emitted from the light source onto the medium. When the optical pickup irradiates an optical storage medium with a light beam, an irradiated portion of the optical storage medium comes to have a different optical property (such as a refractive index) from the other non-irradiated portion of the medium. Such an irradiated portion, of which the optical property has varied, will be referred to herein as a “recorded mark”.
In optical disc technologies, data can be read out from an optical storage medium by irradiating the storage medium with a relatively weak light beam with a constant intensity and detecting the light that has been modulated by, and reflected from, the optical storage medium. On a rewritable optical storage medium, a recording material film, from/on which data can be read and written optically, is deposited by evaporation process, for example, on the surface of a base (which may be either a disc or a long film) on which grooves and lands are arranged. In writing data on a rewritable optical storage medium, data is written there by irradiating the optical storage medium with a pulsed light beam, of which the optical power has been changed according to the data to be written, and locally changing the property of the recording material film.
In a recordable or rewritable optical storage medium, when data is going to be written on its recording material film, the recording material film is irradiated with such a light beam, of which the optical power has been modulated as described above, thereby recording an amorphous mark on a crystalline recording material film. Such an amorphous recorded mark is formed there by heating a portion of the recording material film that has been irradiated with a writing light beam to a temperature that is equal to or higher than its melting point and then rapidly cooling that portion. If the optical power of a light beam that irradiates the recorded mark is set to be relatively low, the temperature of the recorded mark being irradiated with the light beam does not exceed its melting point and the recorded mark will turn crystalline again after having been cooled rapidly (i.e., the recorded mark will be erased). In this manner, the recorded mark can be rewritten over and over again. However, if the power of the light beam for writing data had an inappropriate level, then the recorded mark would have a deformed shape and sometimes it could be difficult to read the data as intended.
To read or write data from/on an optical storage medium, the light beam always needs to maintain a predetermined converging state on a target track. For that purpose, a “focus control” and a “tracking control” need to be done. The “focus control” means controlling the position of an objective lens along a normal to the disc surface so that the focal point (or at least the converging point) of the light beam is always located on the target track. On the other hand, the “tracking control” means controlling the position of the objective lens parallel to the surface of the optical storage medium and perpendicularly to the track so that the light beam spot is always located right on the target track.
In order to perform such a focus control or a tracking control, the focus error or the tracking error needs to be detected based on the light that has been reflected from the optical storage medium and the position of the light beam spot needs to be adjusted so as to reduce the error as much as possible. The magnitudes of the focus error and the tracking error are respectively represented by a “focus error (FE) signal” and a “tracking error (TE) signal”, both of which are generated based on the light that has been reflected from the optical disc.
A conventional apparatus that performs a read/write operation on a write-once or rewritable storage medium such as an optical disc reads data that has been just written and compares the data written to other data stored there in order to detect an error, if any, lest the write operation should fail due to a defect on the storage medium.
Such a verify operation is often performed in a unit at which a constant write or transfer rate can be maintained, not every time a write operation has been finished. That is to say, every time the disc has turned to a predetermined degree, the write operation is suspended, a track jump is made to return to the previous location, that portion on which data has just been written is scanned to detect any error, and then a track jump is made once again to move to a different area and write the next data there. And this series of operations is carried out over and over again. That is why although reliability can be certainly ensured in this way for the data that has been written, it takes a longer time to get the write operation done.
If any error is detected when the data that has just been written is read, then the write operation is retried on another area, not the area on which the write error has occurred. On an optical disc, a set of data and its ID information are stored on the basis of a unit area called a “sector”. Thus, the data that has been written with an error on a sector is rewritten on another sector (which will be referred to herein as a “replacement sector”).
A conventional read/write drive that makes such data correction is disclosed in Japanese Patent Application Laid-Open Publication No. 59-113509 (which will be referred to herein as “Patent Document No. 1” for convenience sake), for example.
Lately, as candidate read/write drives that can save and archive data for a long time in order to meet the rising demand for storing a huge size of data with as high reliability as possible, proposed are an apparatus that uses a so-called “optical tape”, which is a kind of an optical storage medium in a tape shape, and an apparatus that handles a combination of multiple optical disc drives at the same time.
Such a read/write drive to process a huge size of data should not only write and transfer data at sufficiently high rates but also keep the reliability of the written data as high as possible.
Nevertheless, for a storage medium with a low degree of random accessibility such as the tape medium, it is difficult to increase the write rate as long as such a method of writing data and checking the data just written time-sequentially over and over again is adopted as in the conventional optical disc drive.
Thus, to meet such a demand, a so-called “DRAW (direct read after write) technique for performing a write operation and a read operation for verification purposes at the same time has been proposed.
A read/write drive that adopts such a DRAW technique is disclosed in Japanese Patent Application Laid-Open Publication No. 63-249941 (which will be referred to herein as “Patent Document No. 2” for convenience sake), for example.
FIG. 17 illustrates an exemplary arrangement for an optical pickup as disclosed in Patent Document No. 2.
As shown in FIG. 17, the optical system of this optical pickup includes a light source 110, a diffraction grating 111, a detector lens 102, a beam splitter 103, a quarter-wave plate 104, a condenser lens 105, an objective lens 107 and a photodetector 101. The light emitted from the light source 110 gets diffracted by the diffraction grating 111 and split mainly into a zero-order diffracted light beam and ±first-order diffracted light beams. In FIG. 17, the ±first-order diffracted light beams that have left the diffraction grating 111 are schematically indicated by the two arrows. All of the zero-order diffracted light beam and ±first-order diffracted light beams are reflected by the beam splitter 103, transmitted through the quarter-wave plate 104 and the condenser lens 105, and then reflected by a mirror 106. In FIG. 17, only the optical path of the zero-order diffracted light beam is shown for the sake of simplicity. The light beams that have been reflected by the mirror 106 then enter the objective lens 107. Finally, the zero-order diffracted light beam and the ±first-order diffracted light beams are condensed by the condenser lens 107, thereby forming three condensed beam spots (that are a main spot and two sub-spots) on the same track on the optical storage medium 108.
In this description, the zero-order diffracted light beam will be referred to herein as a “main beam” and the ±first-order diffracted light beams as “sub-beams”, respectively.
FIGS. 18(a) and 18(b) are respectively a plan view and a cross-sectional view illustrating the arrangement of light beam spots that are formed by the condensed main and sub-beams on the optical disc 108, which has lands 200 and grooves 210.
In the example illustrated in FIG. 18, the main beam spot formed by the zero-order light beam is a write beam spot, which is used to write a signal on the optical disc 108. On the other hand, the two sub-beam spots formed by the ±first-order light beams are read beam spots, which are used to scan a recorded track. The diffraction grating 111 is designed so that the diffraction efficiency of the ±first-order light beams becomes lower than that of the zero-order light beam. That is why the intensities of the ±first-order light beams are so much lower than that of the zero-order light beam that data that has been written with the zero-order light beam is never erased or altered even when irradiated with the ±first-order light beams.
The main beam spot formed by the zero-order light beam and the two sub-beam spots formed by the ±first-order light beams are located on the same track. In the example illustrated in FIG. 18, as the optical disc 108 turns, the beam spot moves on the land 200 of the optical disc 108 in the direction indicated by the arrow a. More specifically, one of the two sub-beam spots, which is formed by the +first-order light beam, moves behind the main beam spot formed by the zero-order light beam and reads the recorded mark. Meanwhile, the other sub-beam spot formed by the −first-order light beam moves ahead of the main beam spot formed by the zero-order light beam, and its reflected light includes no information about the recorded mark.
These light beams are reflected from the optical disc 108, transmitted through the optical system, and then incident on a photodetector 101, which detects the reflected light of the main and sub-beam spots.
FIG. 19 illustrates the arrangement of photoelectric transducers (photodiodes) in the photodetector 101.
The quadruple photodiode 121 shown in FIG. 19 receives the zero-order light beam (i.e., the reflected light of the main beam spot). The magnitude of astigmatism produced by the detector lens 102 shown in FIG. 17 changes with the degree of defocusing. That is why the photodiode 121 detects not only the focus signal but also the tracking error signal by push-pull method as well.
On the other hand, the photodiodes 122 and 123 receive reflected light of the sub-beam spots 1 and 2, respectively.
The light source 110 emits a light beam that has been driven with a modulation signal in order to record a mark on the optical disc 108. The intensity of the light beam is modulated so as to include either only one pulse or a series of multiple pulses so that a number of marks with various lengths are recorded on the disc 108.
Naturally, the read beams that have been emitted from the same light source 110 have also gone through that modulation. That is why the reflected light of the sub-beam spot that moves behind the main beam spot has a signal component, to which a variation in reflectance caused by a recorded mark and a variation in the quantity of light due to the modulation of light have been added. Meanwhile, the other sub-beam spot moves ahead of the main beam spot through an unrecorded portion, and therefore, its reflected light has not been affected by the variation in reflectance caused by the recorded mark. Consequently, only a signal representing a variation in the quantity of light due to the modulation of the light by the light source 110 is obtained from the reflected light of the sub-beam spot that moves ahead of the main beam spot.
For that reason, by calculating the difference between the two signals representing those two sub-beams, a read signal (i.e., a monitor signal for verification purposes) can be obtained.
By adopting the DRAW technique, even an apparatus that processes a storage medium with big storage capacity but a low degree of random accessibility such as an optical tape can also perform the verify operation while continuing the write operation. Consequently, a system that achieves even higher write and transfer rate and ensures a good deal of reliability is realized. And such a DRAW technique is also applicable effectively to an apparatus that uses multiple optical disc drives in combination.
As for the DRAW technique described above, however, the following respects need to be considered.
First of all, as already described for the example of the prior art, in order to realize a simple and low cost OPU (optical pickup unit) including multiple optical pickups to be built in an optical tape read/write drive, for example, structurally it is preferred to split the light emitted from a single light source into a read beam and a write beam. In that case, however, a write modulated signal will get superposed on a signal generated by the read beam, and therefore, the write modulated signal component should be canceled from the read signal as is done in the example of the prior art.
Meanwhile, even a read/write drive that is ordinarily used for archival purposes should presumably rewrite the data stored. In such a situation, it is preferred that a proper read signal be able to be obtained even while the operation of overwriting something on data already written is being performed.
Furthermore, in a system such as an optical tape read/write drive, the tracking direction as viewed from an optical pickup could possibly be bidirectional instead of unidirectional. Even so, the system should work with as good stability as in a situation where the tracking is carried out in one direction.
The optical read/write drive that has been described as an example of the prior art can cancel the write modulated signal component from the read signal only when one of the two sub-beams is scanning an unrecorded area. That is why such an optical read/write drive still has problems when it comes to overwrite and bidirectional operations.
It is therefore an object of the present invention to provide an optical pickup and optical read/write drive that can read a signal with good stability for verification purposes even when an overwrite operation should be performed on an area where data has already been written.
Another object of the present invention is to provide an optical pickup and optical read/write drive that can operate with as good stability as always even if the optical storage medium changes its traveling direction.